In a silicon carbide semiconductor device in which carriers travel in a vertical direction through a silicon carbide substrate when in operation, a device resistance is decreased by reducing the thickness of the substrate. For further decrease in resistance, there has been a need for a silicon carbide semiconductor device manufactured using a substrate in the form of a thin plate.
In a method of manufacturing a silicon carbide semiconductor device which includes the step of providing a substrate in the form of a thin plate, it is desirable that a back surface structure of the semiconductor device is formed after a front surface structure thereof is formed and that the substrate is thinned during the formation of the back surface structure for the purpose of reducing the number of steps for the thin substrate. For this reason, the back surface structure is formed after the front surface structure is formed. In the step of forming the back surface structure of the silicon carbide semiconductor device, it is however necessary that annealing is performed on only the back surface of the silicon carbide semiconductor device at a high temperature of approximately 1000° C. to form an ohmic electrode, with the from surface thereof maintained at a low temperature.
A method of forming an ohmic electrode on a silicon carbide semiconductor substrate by means of laser annealing is disclosed, for example, in Patent Literature 1. In such a manufacturing method, a metal layer is deposited on a semiconductor substrate made of silicon carbide, and is thereafter irradiated with laser light, so that the metal layer is heated. This forms a layer of an alloy of the metal layer and the semiconductor substrate at an interface therebetween. The alloy layer formed in this manner has excellent ohmic properties with respect to the semiconductor substrate, and is therefore expected to be used as the ohmic electrode.
Unfortunately, when oxygen is present in a laser light irradiation atmosphere, the metal layer is oxidized during the irradiation with the laser light, so that a good alloy layer cannot be formed. As a result, ohmic characteristics of the alloy layer with respect to the semiconductor substrate are degraded. To solve such a problem, a manufacturing method has been proposed in which a permeable film of SiO2, CaF2, ITO and the like is deposited on the metal layer formed on the semiconductor substrate, and in which electromagnetic waves such as laser are directed through the permeable film onto the metal layer to heat the metal layer, thereby forming the alloy layer (for example, Patent Literature 2). In this manner, the laser annealing through the permeable film suppresses the oxidation of the metal film due to ambient oxygen during the irradiation with the laser light.